Monitoring of fetal heart rate trends through the use of an EKG electrode has long been used as an indicator of fetal well-being during labor and delivery. Increased awareness of serious adverse short and long term effects of fetal oxygen starvation (hypoxia) has resulted in a significant increase in the number of babies being monitored in this manner during birth.
Interpretation of heart rate trends in the fetal EKG record is subjective and a skill which requires substantial experience to acquire and maintain and has not been consistently reliable, especially in settings away from the high maternity rate urban areas.
In interpreting heart rate trends in the fetal EKG record, the physician attempts to infer the adequacy of oxygenation in the fetus. This technique is thus indirect and less than satisfactory since it is only after oxygen starvation has been occurring for some time that it is reflected in the EKG record. Also, the record itself is subject to non-deleterious phenomena such as uterine contractions. It is estimated that errors in interpretation of heart rate trends currently yields 50% false positives (conclusion that fetal distress is present when it is actually not) and 20% false negatives (instances where fetal distress is present but was not recognized).
There is an important difference in oximetry and pulse oximetry, oximetry referring to a general measurement of oxygenation of blood and tissue. In contrast pulse oximetry is an established method of determining in real time the relative oxygen saturation of arterial hemoglobin, and is used routinely in surgical suites and intensive care environments. Pulse oximetry provides an immediate and objective measure of oxygenation and requires little technical knowledge or interpretive expertise for its application.
In the opinion of some fetal physiology experts pulse oximetry is a superior technique for monitoring fetal status than pH since pulse oximetry provides real time monitoring of oxygen sufficiency and immediate corrective measures can be taken. If such insufficiency has persisted long enough for it to be reflected by a lower pH, it is likely that some neurological damage has already occurred.
U.S. Pat. No. 4,294,258 to Bernard discloses a measuring head for measuring an ionic or physico-chemical activity, notably pH in a part of, for example, the head of an unborn infant. The FIG. 15 embodiment includes a pair of hollow spiral claws. One of the claws contains a diaphragm sensitive to ionic activity while the second claw contains an extension of a KCL electrode.
Hochberg et al, U.S. Pat. No. 4,658,825, discloses a fetal probe having a single spiral needle containing a pair of optical fibers that are connected to a light source and light sensor exterior of the probe for monitoring EKG and a select chemical condition such as pH. Alternately a second spiral needle could be provided with one needle being used for pH and the other for EKG. The fiber pair carries light to and from the interior of the needle to detect color changes in a pH sensitive dye within the needle, it being stated that body fluids are allowed through a window in the needle and to an ion permeable dye containing membrane within the needle and adjacent to the needle distal end. There is no indication that the light is ever intended to leave through the window.
The Hochberg probe and other probes use pH as an indicator to imply an insufficient oxygen supply, this method being indirect. The logic used is that if the pH has decreased (become acidic), then the concentration of CO.sub.2 has increased. If the concentration of the CO.sub.2 is too high, then circulation/exchange at the placenta is compromised. If that is true, then oxygen supply is also likely insufficient.
Hon, U.S. Pat. Nos. 4,321,931 and Re. 28,990, disclose a conducting helix used as an electrode for use in monitoring fetal EKG.
U.S. Pat. No. 4,281,645 to Jobsis discloses a non-invasive structure and procedure for monitoring a state of metabolic activity in a body organ. That is, this patent is directed to the use of the optical characteristics of the enzyme cytochrome A to monitor cellular oxidative metabolism within an organ such as the heart, brain, or kidney by passing multiple wavelengths of visible and infrared light through the organ. The transmitters are being indicated as being on one side of the organ and the receivers on the opposite side of the organ, for example opposite sides of the head or chest.
Hulka, U.S. Pat. No. 4,537,197 discloses a fetal probe having a suction cup with a first aperture opening through its inner surface through which light from an optical fiber is transmitted to the fetal brain and an adjacent aperture through which the transmitted light is returned through an optical fiber. The light is to penetrate the skull to a depth where enzymatic activity associated with normal oxygenation in brain cells will be evident.
U.S. Pat. No. 3,973,555 to Moller discloses an electric cell assembly having an electrode that can be introduced and anchored in the living tissue of a fetus. The assembly is for measuring pH and includes a spiral to anchor the device.
Farrar et al, U.S. Pat. No. 4,281,659, discloses a probe for fetal monitoring. The disclosure is primarily directed to pH, however it is indicated that the arrangement is compatible with other measuring functions such as fetal ECG, pO.sub.2 etc. The probe includes a base that threading mounts a probe and two spiral electrodes that serve to hold the base in place on the fetal skull.
Aaronoudse, J. G. "Subcutaneous Oxygen Tension Inthe Fetal Scalp During Labour Continuous Monitoring With A Needle Electrode", British Journal of Obstetrics and Gynaecology, 1981: 88: 517, discloses a pair of spirals for attaching the assembly to a fetal skull and a generally straight oxygen needle for measuring subcutaneous Po.sub.2.
Takayama, U.S. Pat. No. 3,822,695, discloses a catheter system for simultaneously obtaining measurements of the blood pressure and the percentage content of oxygen contained in the blood within a blood vessel by inserting one end of an optical fiber bundle within a catheter into the blood vessel. Infrared rays and red light are passed through a catheter and there is individual detection of the amount of light reflected from the carboxyhemoglobin and oxyhemoglobin.
Isaacson, U.S. Pat. No. 4,773,422, discloses pulse oximeter apparatus for non-invasively measuring and indicating the percentage level of various constituents in arterial blood. Light of a plurality of separate wave lengths is sequentially passed through a portion of the body.
In order to overcome problems encountered with various types of prior art probes and to provide improved apparatus to simultaneously monitor fetal EKG and directly measure arterial hemoglobin oxygen saturation of a fetus during labor and delivery using the techniques of pulse oximetry, this invention has been made.